fix: handle edge case with 0 occupied electrons in a specific spin ch…

[bsplu]

When calculating a one-electron system, such as a hydrogen atom, there can be cases where one spin channel has 0 electrons occupied. In such scenarios, the program currently throws a None type error due to the following code:

let mut occ_s = occ.get(i_spin).unwrap()[0..homo_s + 1]
    .iter()
    .map(|occ| occ.sqrt())
    .collect::<Vec<f64>>();

To resolve this, the homo_s value is now checked, and if it is None, an empty vector is returned. This prevents the error and ensures the program handles one-electron systems gracefully.